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New Soybeans With Seed Rot Resistance Identified

The fungus Phomopsis longicolla is largely to blame for a disease called Phomopsis seed decay (PSD) that has claimed more than 5 million bushels of U.S. soybeans each of the past 5 years. The seed disease is most problematic in midwestern and southern states.

Control strategies used by farmers have been inconsistent. These include rotating soybeans with corn or wheat (nonlegume crops on which the fungus can’t grow), treating seed with fungicides, and tilling the soil to disrupt spore dissemination. The ideal defense is to plant resistant varieties.

To that end, Agricultural Research Service plant pathologist Shuxian Li is coordinating a 3-year project out of Stoneville, Mississippi, to screen for PSD resistance in hundreds of soybean germplasm accessions, breeding lines, and commercial cultivars collected from around the world. “Resistant varieties can provide protection for soybean producers at no additional cost beyond the price of planting the seed,” notes Li, in ARS’s Crop Genetics Research Unit.

Her efforts to identify resistant sources kicked into high gear in April 2009 following a grant from the United Soybean Board (USB). Li’s collaborators on the project are Pengyin Chen and John Rupe, professors at the University of Arkansas in Fayetteville, and Allen Wrather, a professor at the University of Missouri in Portageville.

The USB grant expands on prior field trials the team had conducted since May 2007 in Mississippi and Arkansas that identified several promising PSD-resistant soybean lines from commercial varieties provided by Mississippi State University collaborators and plant introductions from the USDA Germplasm Collection. Typically, the resistant lines identified from this research showed little or no incidence of PSD and had a high germination rate with strong vigor. Additional screening using local strains of P. longicolla will also be conducted on soybeans from other sources (including 28 countries).

To expedite their research, Li is developing new and fast screening methods to identify sources of PSD resistance and map resistance genes. The team is also identifying DNA markers associated with the expression of these resistance genes in mapping populations of offspring plants derived from cross-breeding.

Once the markers have been validated, the team will make them publicly available for use in marker-assisted selection, an approach that will save soybean breeders considerable time and expense in developing elite commercial cultivars for growers.—By Jan Suszkiw, Agricultural Research Service Information Staff.